You'll need a crank puller to take the crank arms off, and if it's a cartridge BB (most are unless your bike is old) you'll need a BB tool. Which tool you'll need will depend on the type of bottom bracket. Shimano is the most popular, followed by Campagnolo. External BBs (large rings on the outside of the BB shell) and ISIS drive BBs also have their own tools. These tools fit into the splines on the BB, letting you turn the tool and BB with a wrench.